In a float process of manufacturing a ribbon of glass, molten glass is poured out on a molten tin bath and allowed to flow out. If no forces are applied to the glass, it will flow to an equilibrium thickness of about one-quarter of an inch. However, in order to manufacture thinner glass by the float process, it is necessary to apply stretching forces to the ribbon to draw it down to a thickness less than equilibrium, for example, 0.095 inches.
As is well known in the art, the principal stretching force is applied along the length of the ribbon of glass being formed in a float chamber in order to draw it down to thicknesses less than equilibrium thickness. This stretching force intensifies the drawlines which are visible upon close inspection of the finished float glass. These drawlines are readily visible when one skilled in the art looks through the glass. They can most readily be seen when the glass is placed at an angular relationship with the scene viewed. The drawlines are observable as slight, essentially parallel optical distortions in the appearance of the subject viewed.
U.S. Pat. No. 3,486,673 issued to J. G. Madge for a Method of Cutting Glass, describes generally a method for cutting a glass ribbon manufactured in a float glass operation into individual glass brackets. In order to produce such brackets, the glass ribbon must be cut across its width and along its length. The cross-cuts are made by cross-cutting machines which are generally of complex design because the machine must move in coordinated fashion with the ribbon so that it may compensate for the ribbon's movement in a direction perpendicular to its cutter travel path.
Cuts are made along the length of the ribbon of glass by machines which have been called cord-wood cutters. These stationary machines are of much simpler design because the glass ribbon can move relative to them and because the direction of cut parallels the direction of glass travel.
It has been customary in the industry to use the cross-cutting machines to cut the glass ribbon initially into large individual blocks of glass. Thereafter, the large individual pieces of glass are cut in the opposite direction by the cord-wood cutters into a plurality of oblong glass brackets. In this situation, the most cuts are made on the less complex cord-wood cutter; thus, the drawlines in the glass ribbon, which lie parallel to the direction of movement of the glass, will also lie parallel to the greatest dimension or length of the glass bracket.
When these glass brackets, having the drawlines running along the greatest dimension, are cut into windshield shapes, paired with another bracket to form a pair of glass brackets for a windshield, bent as a paired unit and laminated as a paired unit to form a windshield, certain optical problems can develop. In particular, I have found that the optical problem which the disclosure addresses is one in which the drawlines in the completed windshield are conventionally aligned in the horizontal direction. This occurs because the largest dimension of the glass bracket, and the one to which the drawlines lie parallel, is the dimension which winds up extending across the windshield essentially from one side of the windshield to the other side.
The alignment of the drawlines can result in recognizable distortions of the true external scene when the viewer looks through the windshield with the vehicle in motion. This is because the essential up and down action of the moving vehicle relative to the occupant and to the scene causes the line-of-sight between the viewer (occupant) and the scene to pass successively through a variety of adjacent places in the interposed windshield. These places may alter the scene more or less depending on the presence and degree of floatline distortion existing at each of the (instant) light paths through the windshield.
Such distortions are generally minimal and not generally detected by those who are unskilled in the art. However, they can be detected by skilled artisans and even by observant lay people. It is our desire to produce the very best quality windshields, recognizing that drawlines are inevitably present in some degree in glass manufactured by the float glass process.
I have discovered that if the drawlines of the float glass ribbon are oriented in the windshield in the vertical direction, the eyes of the driver and/or passenger in the vehicle are not moved across the drawlines upon up and down motion, but are moved essentially parallel thereto. This action substantially reduces to a skilled artisan the distortion of the windshield. It is more difficult to make the windshield in this manner because the machine which cross-cuts the glass ribbon now has to be used to make the greater number of cuts because the large dimension of the glass bracket will be perpendicular to the center line of the glass ribbon rather than parallel thereto. This is a more difficult and more costly operation to make the greater number of cuts on the cross cutting machine rather than a cord-wood machine which is the conventional practice. It is additionally more difficult because the maximum available width of the glass ribbon in a given floatline installation may be such as to constitute a constraint on its capability of yeilding efficient full lengths or multiples thereof across the (ribbon) direction of the motion of the glass as proposed herein.